css3/html5 support necessary?

I am working on creating a css3/html5 showcase on my blog. I use a theme that doesn’t explicitly support either css3 or html5 but it was made for WordPress 2.9 (works on/ running 3.0). Without changing anything on the backend I tried to create a post with a <div> styled using some CSS3 elements and -moz/-webkit elements. The output wasn’t promising as neither the css3 nor the -moz/-webkit rendered. I guess at this point my question is: Does the theme have to directly support css3 in order for it work? It doesn’t use the html5 doctype by default, but I’m sure I could change this…don’t think it would fix it though. I feel like I am missing something (it is late here..). Any help is greatly appreciated.

PS. The code is formatted correctly, there are no typos or missed punctuation ect.